This application claims the priority of German Patent Application Serial No. 199 07 471.2, filed Feb. 12, 1999, the subject matter of which is incorporated herein by reference.
The present invention relates to a process of manufacturing rotationally symmetrical articles, such as rope pulleys, running wheels or the like, from plastic material, in particular cast polyamides.
In Dubbel xe2x80x9cHandbook for the Mechanical Engineering, 18th ed., page U9, chapter 2.2.3, entitled xe2x80x9cSeilrollen und Seiltrommelnxe2x80x9d, reference is made to the provision of rope pulleys made from plastic material, preferably polyamide, for use in mobile cranes. According to another publication, xe2x80x9cPlastics-Handbookxe2x80x9d, vol. VI, entitled xe2x80x9cPolyamidesxe2x80x9d, pages 46 to 49, published by Carl Hanser Verlag, a process is described in which a component A comprised of a catalyst-containing lactam melt, and a component B comprised of an activator-containing lactam melt are mixed and polymerized in a mold. After polymerization and crystallization, the solidified article is removed from the mold and, optionally, mechanically re-dressed. Examples for lactam include xcex5-caprolactam or C-substituted lactams.
Rotationally symmetrical articles, such as rope pulleys, rope sheaves, running wheels or the like are typically manufactured through a centrifugal casting process in which a heated mold rotates at high speed about a horizontal or vertical axis. Raw material melt is poured into the rotating mold and pushed by centrifugal forces against the mold wall to thereby prevent shrink holes and air pockets.
The use of rope pulleys or rope sheaves of plastic in crane construction is advantageous for several reasons: the rope is exposed to smaller loads as a consequence of the enhanced pressure distribution between rope pulley and rope, so that the rope is subjected to reduced wear and thereby accomplishes a significantly longer life. In addition, the manufacture of the rope pulley of plastic material results in weight and cost advantages.
A major problem facing the art in connection with the use of a rope pulley of plastic material is the load to which the hub area (bearing seat) is exposed. For cost reasons and spatial needs (width of the bearing), smallest possible compact bearings are used. At elevated temperatures, which in summer may rise in extreme cases up to 50xc2x0 C., the modulus of elasticity of polyamide drops considerably (20xc2x0 C.: 2800 N/mm2; 50xc2x0 C.: 1400 N/mm2), so that the bearing seat begins to yield at the edge when the rope is pulled at a slant. This results in excessive pressure at the edge of the bearing seat. The slanted disposition of the rope pulley causes a shift of the load, thereby increasing the slanted pull until the rope pulley fails.
German Pat. No. 44 27 206 describes a fiber composite in the form of mats, rovings, fabric, dressing plate or needle felt, which is made of glass fibers or plastic fibers. The fiber composite is fixed in place in a mold before polymerization to realize defined deposits of the fiber composite in the finished article, thereby improving the mechanical properties. Practice has shown, however, that this process is highly uncertain and complicated. The polymerization is adversely affected by the ingress of moisture and the presence of coatings on the fibers. The anticipated increase in strength, in particular in those areas of the articles where it is demanded, is not realized in view of a lack of reproducibility.
German Pat. No. 1, 936,822 describes a process for manufacturing plastic articles with adjustable mechanical properties, by embedding particles of polytetrafluoroethylene (PTFE) in the plastic material. This process suffers shortcomings because a required complete and even distribution of the PTFE particles in the mass of plastic material is difficult to realize so that the surface of the article being made as well as any random interface are uniformly infiltrated. The reason for this difficulty resides in the comparably small viscosity of the melted plastics mass so that PTFE particles, which have a high specific weight, descend fairly rapidly in the melted plastics mass and accumulate in one spot. Attempts to solve this problem include the admixture of particles of a thermoplastic plastic to the particles of PTFE in a preliminary stage such that the mixture, when the plastic is melted, has the density of the pourable polymer. Equalizing the difference in density prevents a demixing as a result of descending PTFE particles in the pourable polymer. After cooling down, the mixture of plastic and PTFE particles is granulated, and the granulate is blended to the pourable polymer in accordance with the desired mechanical properties.
It is known to add PTFE purely for tribological reasons to positively influence the friction and wear behavior. The addition of PTFE results, however, in all cases to a decrease of the strength. This is true even when using a different plastic material as intermediate carrier for the PTFE particles. Adjustment of desired mechanical properties is therefore unattainable through addition of PTFE.
In a further example, German Pat. No. 1, 936,822 describes a method in which a first layer has begun to polymerize in a mold. Subsequently, a layer, e.g. a fiberglass mat, is placed in the mold before the next layer is poured. The placement of fiberglass as intermediate layer cannot be realized without using fastening members because the own weight of the layer of fiberglass and the mass flow of the next layer will otherwise drag the fiberglass layer downwards. Thus, the fastening members are incorporated in the material during polymerization, thereby complicating the process and significantly adversely affecting the quality of the finished product.
It is thus an object of the present invention to provide an improved process for manufacturing rotationally symmetrical articles, such as rope pulleys, running wheels or the like, from plastic material, obviating the afore-stated drawbacks.
In particular, it is an object of the present invention to provide an improved process for manufacturing rotationally symmetrical articles from plastic material, such as polyamides, which is simple in structure and yet reliable in operation to realize a reproducible increase in strength in selected regions.
These objects, and others which will become apparent hereinafter, are attained in accordance with the present invention by preparing first and second lactam melts of such formulations and composition as to exhibit different mechanical properties, and pouring the first and second melts sequentially into a mold in dependence on the course of polymerization, without interruption of a rotating motion of the mold.
According to another feature of the present invention, filler and/or reinforcing agents are added to the first and second melts. Examples of such filler and/or reinforcing agents include fiberglass, plastic fibers, aramide fibers and natural fibers, laminar talcum, silane-treated wollastonite, talc pellets etc. or glass balls. The addition of such filler and/or reinforcing agents in conjunction with the variation of lactams, activators, catalysts, crystallization accelerators etc. realizes a significantly increased and targeted influence of material and structure with respect to modulus of elasticity, tensile strength, absorption capability etc. A targeted reinforcement of, e.g., the hub area can be accomplished by preparing through admixture of catalysts and activator a defined first xcex5-caprolactam melt at an amount which corresponds to the volume of the article as defined from the outer article edge in radially inward direction to the hub area, pouring the melt into a rotating heated mold, and subsequently stopping the pouring while maintaining a rotation of the mold. At the moment of commencement of polymerization of the melt in the mold, a second xcex5-caprolactam melt, mixed with catalyst and activator as well as reinforcing agents such as glass balls or glass fibers, is poured into the mold for formation of the hub area, whereby this second xcex5-caprolactam melt has increased strength properties in the final stage in comparison to the first xcex5-caprolactam melt.
As a commencing of the polymerization process in the mold is accompanied by a steady increase in viscosity of the lactam melt, a pouring of additional lactam melt results in an intimate and even bond of both melts in the contact area; However, a thorough mixing of both lactam melts is prevented by this procedure. After completed polymerization, the cast article is removed from the mold and mechanically re-dressed in a manner known per se.
According to another feature of the present invention, an increase in strength in selected areas is also possible by placing one or more strength-enhancing and stiffness-enhancing elements in the cast mold before the casting process to form with the added raw material melts a form-fitting and/or force-fitting or cross-linked composite. Suitably, an insert of metal is used which has a degree of strength at least corresponding or exceeding the strength of the reinforced raw material melt. An intimate bond can be realized when roughening the surface of the insert and/or forming the inserts with impressions, grooves or breakthroughs such as slots or bores. A suitable material for the insert includes aluminum or an aluminum alloy because the specific weight is low and the strength is high in comparison with plastic material.
The process according to the present invention is applicable for all types of rotationally symmetrical articles which have annular volume parts of varying strength. Not only can the hub area be reinforced by the process according to the present invention, but also the edge area of the article, e.g. the area of the groove for the rope, i.e. an area that is also subject to substantial loads, can be reinforced by this novel and inventive process.
Through the process according to the present invention, the advantages of using plastic material for manufacturing rotationally symmetrical articles, in particular the low weight and the superior pressure behavior and wear behavior are retained while eliminating the drawbacks, experienced heretofore, in the hub and bearing zone which is subject to substantial loads. In addition, the novel and inventive process, involved here, allows an incorporation of layers of varying materials, e.g. for sandwich panels or the like, when articles or semi-finished products are cast without pressure.